Features of the hydrological regime of the rivers of the Upper Southern Bug

The article examines in detail the hydrological regime of the rivers of the upper reaches of the Southern Bug basin. This part of the Southern Bug river basin is poorly studied, since most scientific publications are focused on studies of the Middle and Lower Bug. It is interesting that at the beginning of the 20th century, 9 hydrological posts operated within this territory, 5 of which were actually on the main river and 4 on small tributaries. Unfortunately, most of them were closed and hydrological information was lost, so the analysis was carried out only on the basis of two operating observation points on the Southern Bug and Ikva rivers. The paper presents an initial assessment of the hydrological regime based on average, maximum and minimum runoff indicators, determines cyclical changes in fluctuations in the water flow of the main river, and calculates the correlation coefficients of the average annual runoff of the Southern Bug and Ikva rivers for the adjacent observation period (1964-2022). As a result of the analysis, it can be stated that the hydrological regime of the Southern Bug River, as well as the Ikva River, is significantly influenced by the regulation of flow by ponds and small reservoirs. An interesting feature is also that for the Southern Bug River, as for the Ikva River, the asynchrony of trend lines on the graphs of the combination of average annual levels and discharges is characteristic, i.e. over a long period of observations for both rivers, an increase in the levels of Naverage is accompanied by a decrease in the discharge of Qaverage. The trends of maximum discharges are also similar, the amplitude of which has fallen sharply after 1980, i.e. over the past almost 40 years, catastrophic floods have not been observed on both rivers. Summer low flow over a long period of time has shown a tendency to increase in levels against the background of more or less stable flows, and winter low flow is also characterized by an increasing trend line for levels and relative stability for flows. Another feature of the minimum flow of the Southern Bug River is the synchronous cyclicality in the fluctuations of flows of both summer and winter low flow. The period from 1991 to 2005, both for the open channel and for the closed one, is characterized by the largest flow values compared to previous years and the current period until 2022. That is, during 1991-2005 on the Southern Bug River, both summer and winter low flow were not long and clearly expressed, and the minimum flow was increased, in contrast to the current period, when clear trends are observed to increase the duration of low-water phases during the year. At the same time, the analysis of the graphs of cyclic changes in average water discharge for both rivers is not balanced and coincides only at the peaks of the high-water phases. The duration and amplitudes of the cycles do not coincide at all, and the low-water phases are almost not synchronized. Based on these graphs, it was possible to determine the optimal periods for calculating the correlation coefficient between the average annual discharges on the Ikva and Southern Bug rivers. This allowed us to assume that since 2000, the flow fluctuations on both rivers have become almost synchronous, the correlation coefficient is 0.88.